In a wireless mobile communications system, a power amplifier is a key component that determines a lifetime of a battery in a mobile terminal, e.g., a conventional CDMA mobile handset. Thus, the power amplifier is required to be of a high efficiency characteristic in order to increase the lifetime of the battery. Since the power added efficiency of the conventional power amplifier is highest when an output power thereof is at its highest level, it lowers as the output power backs off at the highest level, e.g., 30 dBm. However, the conventional power amplifier usually operates at a relatively low output power ranging from, e.g., −15 to 15 dBm. Therefore, there have been proposed various schemes capable of improving the power added efficiency of the power amplifier at such low output power range, by way of increasing a quiescent current when an output power is high and decreasing the quiescent current when the output power is low.
Such methods include a technique for controlling VCC (DC supply) or VB (base bias voltage) of a bias circuit, and a dual bias control technique for controlling both VCC and VB. All of these techniques adopt a DC-to-DC converter, which requires a DSP (digital signal processor), or an RF (radio frequency) coupler together with an envelope detector for the control thereof, specifically the RF coupler together with the envelope detector being adopted in case the control is implemented in an RF range.
The technique for controlling VCC mentioned above is directed to reduce DC power consumption when the power amplifier is in the low output power mode. To be specific, VCC is reduced in the low output power mode but increased in the high output power mode by using the DC-to-DC converter to thereby improve the efficiency of the power amplifier.
The technique for controlling VB mentioned above accomplishes the power added efficiency improvement by way of adopting the DC-to-DC converter to control VB. To be more specific, in the low output power mode, the DC-to-DC converter reduces the bias current and thus, decreases the DC power consumption, while, in the high output power mode, the DC-to-DC converter increases the bias current.
The dual bias control technique increases the power added efficiency of the power amplifier by simultaneously controlling both VCC and VB in a manner described above.
All of the above-mentioned conventional techniques adopt the DC-to-DC converter to control the DC power consumption depending on the output power mode of the power amplifier. These conventional techniques, however, have a drawback in that it is very difficult to install such components as the RF coupler/envelope detector and the DC-to-DC converter within a highly miniaturized module of power amplifier having a size of, e.g., 6×6 mm2. Thus, it may be desired to develop a method for increasing the power added efficiency of the power amplifier when the output power is low as well as maintaining a high linearity of the power amplifier when the output power is high without an additional component such as the DC-to-DC converter.